The present invention relates to a method and apparatus for recovering uranium and/or related values from an ore, either at the surface or in situ. More particularly, the present invention relates to a method and apparatus for recovering uranium which includes means for protecting the ion-exchange resins used in the recovery operation from oxidative degradation caused by contact with hydrogen peroxide.
In a typical uranium recovery operation, uraniumbearing ore (either mined or in situ) is contacted with a leach solution which dissolves the uranium values from the ore. The pregnant leach solution is then flowed through one or more columns filled with a strong base, anionic, ion-exchange resin which adsorbs the uranium values from the leach solution. When the resin in a column is sufficiently loaded with uranium values, the flow of leach solution is switched to another column and a second solution or eluant is flowed through the loaded column to desorb the uranium values into the eluant, now called eluate when loaded with uranium values. Due to the eluant composition, the concentration of uranium value in the eluate will be greater than was the concentration in the leach solution. The eluate is then processed to precipitate and recover the uranium values. The barren eluate, now stripped of uranium values, is made up with chemicals to form fresh eluant for recycle in the operation.
In known commercial operations where the leach solution contains carbonate ions, the ion-exchange columns are commonly eluted with an eluant of sodium carbonate/sodium bicarbonate/sodium chloride. To precipitate the uranium values from this type of eluant, the pH of the eluate (i.e., pregnant eluant) is first adjusted to between 1 and 2 to decompose the uranyl carbonate complex in the eluate causing CO.sub.2 to be expelled from the eluate. Hydrogen peroxide (H.sub.2 O.sub.2) is added to oxidize the UO.sub.2.sup.+2 to UO.sub.4. XH.sub.2 O which, in turn, precipitates as "yellowcake" and is recovered as the desired product of the operation. The solution is readjusted to pH of 3 to 5 to assure complete precipitation of yellowcake. The barren eluate, now stripped of its uranium values, is made up with more chemicals to make fresh eluant for future use in the operation.
However, in the above reaction, the exact stoichiometric amount of hydrogen peroxide is sometimes difficult to maintain and excess hydrogen peroxide may inadvertently be added during the precipitation step. Further, excess hydrogen peroxide may be deliberately added in some instances to insure the complete precipitation of a high quality yellowcake product. For whatever reason, the addition of excess hydrogen peroxide in the precipitation step will result in the presence of hydrogen peroxide in the barren eluate which eventually ends up in the fresh eluant. Unfortunately, the quaternary amine, anionic resins used in this type of operation is highly susceptible to oxidative degradation when contacted by hydrogen peroxide. Upon oxidation, the resins lose their functional group and structural integrity and crumble, thereby severely reducing both the effectiveness and the operational life of the resin. Since these resins are a critical part of the operation and are a very high cost item, it is obvious that a need exists for protecting these resins from accidental contact with hydrogen peroxide.
To function satisfactorily in a typical, commercial leach operation, the means used for protecting the resins should (1) be effective at ambient temperatures to lower the hydrogen peroxide to operating levels of approximately 1.about.2 parts per million; (2) add no additional reagents to the elution circuit which might yield harmful by-products; and (3) be inexpensive, easy to operate, and require no tight control.